I use the Blood Dragon Nightmare Wings, they fit very very nicely on the back of the Tyrant (see my blog for pics).

Hive Fleet Fuzzy Bunny: Lots and lots of points of Nids (4000pts+)
The Chapter of the Damned: 5500pts of Dark Angels
The Children of Fulgrim: 5-6000pts of Emperors Children
The Kabal of the Bloodied Claw: 2000pts of Dark Eldar

I'm going to second the vote for Balrog wings. They're the only thing (in my opinion) that is even close to big enough for a hive tyrant. Wings have to be very large in order to be able to support the mass of a tyrant, and only Balrog wings (that I've seen) are close to the type of size needed for them.

Also they're plastic, purchaseable seperately, and easy enough to work with. (I did my conversion with them which turned out decently well).

I have no idea where he got them, perhaps they were scratch-built (which is also perfectly legal), but the owner of my local hobby store gave all of his flying warriors and tyrant wings that looked more akin to dragonfly wings. He also based them flying in a horizontal postiion on long thin lengths of wire. They looked amazing, and I feel the insectoid wings looked better than reptilian wings. Don't have any idea how fluffy it is though.

I also vote for balrog/fell beast wings. I used scratch-built wings, but they're modelled after balrog wings anyhow. They're of an appropriated size to "reallistically*" support a tyrant-sized model flight, and they look incredible.